1. Field of the Invention
The invention relates to an electrostatic lens or an electrostatic lens array. Furthermore, the invention relates to a charged particle lithography system comprising such an electrostatic lens or electrostatic lens array. Additionally, the invention relates to a method of manufacturing an insulating structure for use in an electrostatic lens.
2. Description of the Related Art
Electrostatic lenses are used in charged particle beam columns for use in applications like scanning electron microscopes and lithography apparatus. An electrostatic lens typically consists of conductive plates, generally silicon plates provided with a conductive layer, stacked in a direction of the charged particle beam. The conductive plates may be separated by electrically isolating spacers, e.g. made of glass or other suitable insulating materials. The plates and spacers have aligned apertures enabling a charged particle beam to pass through the plates along a charged particle transmission path. The plates may be charged to enable actions such as focusing, blanking and steering of the charged particle beam.
The electric field that can be applied between the plates depends on the plate material properties and the positioning of the plates with respect to each other. Furthermore, the properties and structure of the insulating barrier between the plates, e.g. electrically isolating spacers made of glass or vacuum, are of importance. If the insulating barrier is stressed beyond its dielectric strength, a sudden transition from an insulating state to a highly conductive state takes place. Such a transition may be in the form of an electric discharge or flashover.
The surface of a solid insulator will often provide a path for such an electrical discharge. If such a discharge occurs in a solid dielectric, typically occurring along a surface of the insulator between the plates, changes along a discharge path caused by the discharge can result in a permanent reduction in the dielectric strength of the insulator. As a result, the insulator is degraded and a high electric field cannot be maintained by the electrostatic lens, reducing the performance of the lens.
Conventional efforts to reduce such electrical discharges between conductive plates rely on increasing the length of the breakdown path along surfaces where electrical breakdown is likely to occur, as compared to the direct, i.e. shortest distance between the plates. Examples of this approach are described in Japanese patent publications JP2000260242 and JP2005214908, where beveled or rectangular recesses are included at the location of connection with an electrode.
However, the requirement to maintain the electrode plates close together limits the ability to elongate the discharge path between the electrodes. Furthermore, merely increasing the surface breakdown path length may be insufficient if high electric fields need to be applied between conductive plates positioned in close proximity to each other, e.g. resulting in 10-50 V/micron of electric field to be covered. Therefore, further design measures are needed to enable electrostatic lenses to operate under these circumstances without a considerable risk of electrical breakdown.